WASHINGTON, District of Columbia (AFP) - Top seed Juan Martin Del Potro advanced with ease on Thursday at the ATP and WTA Washington Open while Olga Puchkova hit a ball into a line judge and defaulted.

Rain on Wednesday postponed the opening matches for Del Potro and Japanese second seed Kei Nishikori, who had first-round byes, and halted two other men's matches at the US$1.76 million (S$2.24 million) hardcourt event.

So facing two matches on Thursday, Del Potro made the most of his time on the court, ripping American Ryan Harrison 6-1, 7-5 in 70 minutes in his first start since losing to top-ranked Novak Djokovic in an epic five-setter, the longest semi-final in Wimbledon history.

"It's going to be my first time to play two matches in one day, but I think I will be ready," said the Argentine.

Del Potro, who showed no signs of a knee injury that nagged him at Wimbledon, booked a third-round date against Australian 14th seed Bernard Tomic.

"I expect a tough match," Del Potro said. "Tomic has a lot of talent and he is going to be a great player very soon."

Del Potro broke Harrison twice to open the match, winning 16 of the first 22 points on his way to a 4-0 lead and broke again to claim the first set in 22 minutes.

Del Potro, a two-time Washington winner and the 2009 US Open champion, did not break again until the final game.

"I played well in the first set and it got humid," Del Potro said. "The sun came out and it was difficult to play. Now I have to recover."

Nishikori, ranked a career-best 11th in the world, defeated American Jack Sock 7-5, 6-2 in one hour and 44 minutes to book a third-round match later with Cypriot Marcos Baghdatis.

"It's not easy, especially with the conditions heavy, not hot but humid, sweating a lot, have to drink a lot," Nishikori said. "But my body is OK so it should be all right."

Nishikori could not recall the last time he played twice in one day.

"I had a great match so I should be ready for the next one," Nishikori said. "Had a good meal after the match, a couple hours to just relax and get a massage and I'll get a good warmup."

On the women's side, the only match halted on Wednesday was Puchkova's.

She trailed Argentina's Paula Ormaechea 3-6, 6-3, 2-1 when play resumed Thursday but surrendered a break and trailed 4-1 in the final set.

Upset at herself after a point, Puchkova struck a ball in frustration and it slammed into the knee of a line judge, injuring the man and prompting the umpire to award the match to her South American foe.

"It's not my favorite way to win a match," Ormaechea said in a Twitter posting.

Puchkova, who had not deliberately fired the ball at the official, walked over to see the line judge as he lay on the court after being struck and apologised to him.

South African seventh seed Kevin Anderson, coming off a runner-up finish last week in Atlanta, broke Australian James Duckworth in the last game of a rain-halted match once play resumed to complete a 6-3, 3-6, 6-4 triumph.

"It was definitely tricky coming out very near the conclusion of the match," Anderson said.

Anderson was set to face American Mardy Fish later to decide a quarter-final spot.

The winner puts himself in Del Potro's path for a potential Friday quarter-final, but Anderson does not expect the South American star to be slowed even after two matches in one day.

"It's definitely tough. It's never easy," Anderson said. "Depends on the length of the matches. Del Potro as an example, I would be very surprised if two matches today would affect him for a match tomorrow.

"Somebody with the level of fitness of the players out here, it won't be a problem."

German third seed Tommy Haas won the other rain-halted match, taking the last set to beat US qualifier Tim Smyczek 3-6, 7-5, 6-3.

"It messes you up for the next day but that's life," Haas said. "When it mattered, I played pretty solid."